The circulatory system is made up of the heart and blood vessels (血管) (blood is a tissue, not an organ). Its function is to transport food and oxygen to all cells in the body and transport waste produced by the body.
Blood
The human body contains about 5 litres of blood. Blood contains:
- Red blood cells (紅血細胞) - transport oxygen around the body from the lungs to tissues throughtout the body and transport waste (CO2) from tissues to the lungs. The oxygen is carried by the haemoglobin as oxyhaemoglobin (氧合血紅蛋白).
- White blood cells (白血細胞) - these help to protect the body against disease. There are two types of white blood cells: phagocytes (吞噬細胞) and lymphocytes (淋巴細胞). Phagocytes kill bacteria by engulfing them. Lymphocytes produce antibodies (抗體) which are carried around the body killing the bacteria (菌) and neutralising the poisons.
- Platelets (血小板) - these are fragments of blood cells and they help the blood to clot to stop bleeding.
- Plasma (血漿) - this is a pale yellow liquid which is largely water. It contains dissolved substances such as glucose, amino acids, salts, hormones, blood proteins and waste products.
Blood vessels
Blood flows continuously around the body, pumped by the heart. The blood travles through blood vessels. There are three types of blood vessel:
- Arteries (動脈) carry oxyhaemoglobin blood away from the heart. The blood in the arteries is under pressure so they have thick muscular walls to withstand this pressure.
- Veins (靜脈) carry deoxygenated blood back to the heart. The blood in the veins is at a much lower pressure so the vein walls are much thinner and less muscular. Veins have valves to prevent the flow of blood in the wrong direction. Veins are usually much closer to the surface of the skin.
- Capillaries (毛細管) are very narrow, thin-walled tubes which join the arteries to the veins. Every organ contains thousands and thousands of capillaries. They have thin walls so that water, oxygen and dissolved foods can diffuse through them to be used by the organ. Waste products can diffuse in the opposite direction.
Circulation of Blood
Heart acts as a pump to force blood to flow. The heart pumps blood through two main arteries and takes blood back from two main veins. Blood enters the right atrium and the right ventricle of the heart deoxygenated and pumped to the lungs through one of the main arteries where blood picks up oxygen and gets rid of carbon dioxide. The blood with oxyhaemoglobin then returns from the lungs to the heart through the left atrium and the left ventricle and flows out the other main artery to other areas:
- to the small intestine where most of food absorbed by blood then to the liver which processes and stores food
- to the kidneys where waste products are removed
- to the rest of the body (brain, muscles etc.) taking food and oxygen from the blood and passing wastes back into the blood
The vena cava (right) vein returns deoxygenated blood (缺氧血) to the heart and the heart pumps the deoxygenated blood through the pulmonary (right) artery to the lungs. The oxygenated blood (含氧血) returns from the lungs via the pulmonary (left) vein and flows out the aorta (left) artery to the brain, gut, liver, kidneys etc.